Vacuum IG units are known in the art. For example, see U.S. Pat. Nos. 5,664,395, 5,657,607, and 5,902,652, the disclosures of which are all hereby incorporated herein by reference.
Prior art FIGS. 1-2 illustrate a conventional vacuum IG unit. IG unit 1 includes two spaced apart glass substrates 2 and 3 which enclose an evacuated or low pressure space 6 therebetween. Glass sheets/substrates 2 and 3 are interconnected by peripheral or edge seal of fused solder glass 4 and an array of support pillars or spacers 5.
Pump out tube 8 is hermetically sealed by solder glass 9 to an aperture or hole 10 which passes from an interior surface of glass sheet 2 to the bottom of recess 11 in the exterior face of sheet 2. A vacuum is attached to pump out tube 8 so that the interior cavity between substrates 2 and 3 can be evacuated to create a low pressure area or space 6. After evacuation, tube 8 is melted to seal the vacuum. Recess 11 retains sealed tube 8. Optionally, a chemical getter 12 may be included within recess 13.
Conventional vacuum IG units, with their fused solder glass peripheral seals 4, have been manufactured as follows. Glass frit in a solution (ultimately to form solder glass edge seal 4) is initially deposited around the periphery of substrate 2. The other substrate 3 is brought down over top of substrate 2 so as to sandwich spacers 5 and the glass frit/solution therebetween. The entire assembly including sheets 2, 3, the spacers, and the seal material is then heated to a temperature of approximately 500xc2x0 C. at which point the glass frit melts, wets the surfaces of the glass sheets 2, 3, and ultimately forms hermetic peripheral or edge seal 4. This approximately 500xc2x0 C. temperature is maintained for from about one to eight hours. After formation of the peripheral/edge seal 4 and the seal around tube 8, the assembly is cooled to room temperature. It is noted that column 2 of U.S. Pat. No. 5,664,395 states that a conventional vacuum IG processing temperature is approximately 500xc2x0 C. for one hour. Inventor Collins of the ""395 patent states in xe2x80x9cThermal Outgassing of Vacuum Glazingxe2x80x9d, by Lenzen, Turner and Collins, that xe2x80x9cthe edge seal process is currently quite slow: typically the temperature of the sample is increased at 200xc2x0 C. per hour, and held for one hour at a constant value ranging from 430xc2x0 C. and 530xc2x0 C. depending on the solder glass composition.xe2x80x9d After formation of edge seal 4, a vacuum is drawn via the tube to form low pressure space 6.
Unfortunately, the aforesaid high temperatures and long heating times utilized in the formulation of edge seal 4 are undesirable, especially when it is desired to use a tempered glass substrate(s) 2, 3 in the vacuum IG unit. As shown in FIGS. 3-4, tempered glass loses temper strength upon exposure to high temperatures as a function of heating time. Moreover, such high processing temperatures may adversely affect certain low-E coating(s) that may be applied to one or both of the glass substrates.
FIG. 3 is a graph illustrating how fully thermally tempered plate glass loses original temper upon exposure to different temperatures for different periods of time, where the original center tension stress is 3,200 MU per inch. The x-axis in FIG. 3 is exponentially representative of time in hours (from 1 to 1,000 hours), while the y-axis is indicative of the percentage of original temper strength remaining after heat exposure. FIG. 4 is a graph similar to FIG. 3, except that the x-axis in FIG. 4 extends from zero to one hour exponentially.
Seven different curves are illustrated in FIG. 3, each indicative of a different temperature exposure in degrees Fahrenheit (F). The different curves/lines are 400xc2x0 F. (across the top of the FIG. 3 graph), 500xc2x0 F., 600xc2x0 F., 700xc2x0 F., 800xc2x0 F., 900xc2x0 F., and 950xc2x0 F. (the bottom curve of the FIG. 3 graph). A temperature of 900xc2x0 F. is equivalent to approximately 482xc2x0 C., which is within the range utilized for forming the aforesaid conventional solder glass peripheral seal 4 in FIGS. 1-2. Thus, attention is drawn to the 900xc2x0 F. curve in FIG. 3, labeled by reference number 18. As shown, only 20% of the original temper strength remains after one hour at this temperature (900xc2x0 F. or 482xc2x0 C.). Such a significant loss (i.e., 80% loss) of temper strength is of course undesirable.
In FIGS. 3-4, it is noted that much better temper strength remains in a thermally tempered sheet when it is heated to a temperature of 800xc2x0 F. (i.e., about 428xc2x0 C.) for one hour as opposed to 900xc2x0 F. for one hour. Such a glass sheet retains about 70% of its original temper strength after one hour at 800xc2x0 F., which is significantly better than the less than 20% when at 900xc2x0 F. for the same period of time.
It will be apparent of those of skill in the art that there exists a need for a vacuum IG unit, and corresponding method of making the same, where a structurally sound hermetic edge seal may be provided between opposing glass sheets without at least certain portions of thermally tempered glass sheet(s)/substrate(s) of the IG unit losing more than about 50% of original temper strength. There also exists a need in the art for a vacuum IG unit including tempered glass sheets, wherein the peripheral seal is formed such that the glass sheets retain more of their original temper strength than with a conventional vacuum IG manufacturing technique where the entire unit is heated in order to form a solder glass edge seal. There also exist a need in the art to decrease post-tempering processing time, and to reduce the long time period(s) now believed necessary to form a solder glass edge seal in a vacuum IG unit. It is a purpose of this invention to fulfill one or more of the above listed needs in the art.
An object of this invention is to provide a vacuum IG unit having a peripheral or edge seal formed so that at least certain portion(s) of thermally tempered glass substrates/sheets of the IG unit retain more of their original temper strength than if conventional edge seal forming techniques were used with the solder glass edge seal material.
Another object of this invention is to provide a vacuum IG unit, and method of making the same, wherein at least a portion of the resulting thermally tempered glass substrate(s) retain(s) at least about 50% of original temper strength after formation of the edge seal (e.g., solder glass edge seal).
Another object of this invention is to reduce the amount of post-tempering heating time necessary to form a peripheral/edge seal in a vacuum IG unit.
Yet another object of this invention is to form a hermetic edge seal in a vacuum IG unit by utilizing microwave energy to cure edge seal material. In an exemplary embodiment, glass frit suspended in liquid or solution may be deposited as an edge seal material and thereafter heated via microwave energy in order to form the edge seal of a vacuum IG window unit. In other embodiments, microwave energy may also or instead be used to form a seal(s) (e.g., solder glass seal) around a pump out tube of a vacuum IG unit.
The use of microwave energy (localized or otherwise) in order to form an edge seal enables one or both of the thermally tempered glass sheets/substrates to retain much temper strength because at least certain portions (e.g., central portions) of the glass substrate(s) need not be heated along with the solder glass edge seal material during formation of the edge seal. Moreover, the use of microwave energy in forming the edge seal of a vacuum IG unit can result in reduced processing time as well as a reduced need for capital intensive manufacturing equipment such as ovens, furnaces, or the like.
Another object of this invention is to fulfill one or more of the above listed objects and/or needs.
This invention fulfills one or more of the aforesaid needs and/or objects by providing a method of making a seal of a thermally insulating glass panel, the method comprising the steps of:
providing first and second at least partially tempered glass substrates, with a plurality of spacers therebetween;
forming a seal (e.g., peripheral or edges seal) in contact with at least one of the substrates using microwave energy in a manner so that after the seal has been formed at least certain portions of the first and second substrates retain at least about 50% of original temper strength; and
evacuating a space between the first and second substrates so as to form a vacuum or low pressure area having a pressure less than atmospheric pressure between the first and second substrates.
Certain embodiments of this invention further fulfill one or more of the aforesaid needs and/or objects by providing a thermally insulating glass unit comprising:
first and second at least partially tempered glass substrates spaced apart from one another via at least a plurality of spacers;
a microwave energy-formed hermetic peripheral or edge seal located at least partially between the first and second substrates, said peripheral or edge seal having been formed in a manner such that at least certain portions of the first and second substrates retain at least about 50% of their original temper strength after formation of the seal; and
a space having a pressure less than atmospheric pressure provided between said substrates and sealed off by said microwave energy-formed hermetic peripheral or edge seal.